a. Field of the Invention
The invention relates to a quick-hardening core and molding sand and a method for hardening thereof.
B. Prior Art
Quick-hardening core and molding sands are known based on quartz sand and containing water-glass as a binder and these sands have been hardened by means of blowing them with carbonic acid anhydride.
In the case of molds and cores made of molding sand with addition of various kinds of synthetic resins, hardening is achieved by means of blowing these molds and cores with a catalyst in the form of a gas, e.g. hydrogen chloride, or with a liquid catalyst in the form of a mist, e.g. triethylamine, or by adding the catalyst to the molding sand together with the resin.
Utilization of these molding sands and their methods of hardening have caused numerous troubles as regards the use of highly toxic gases or catalysts, in which case it is necessary to construct special apparatus and very tight devices for the supply of these gases or catalysts.
Moreover, molds and cores produced by means of the known methods require a relatively long time for complete hardening of the sand. This time depends on the size of the core or mold and on the degree of complexity of their shape and, on an average, the time amounts to approximately 4 hours.
Another method for making foundry cores is described in U.S. Pat. No. 3,679,703 and consists in using a copolymer of styrene with acrylic acid. The process of binding the sand is a result of the chemical reaction which occurs after adding acrylic acid and styrene in a ratio of 1 : 4, the water formed during this reaction being removed in a dehydrating operation. This method is highly complicated.
Another method for conditioning foundry sand is described in U.S. Pat. No. 2,491,006 and comprises mixing an appropriate amount of the foundry sand in a mixer at elevated temperature and adding thereto an appropriate heavy petroleum residue. However, this does not confer to thus obtained moulding or core sand thermohardening properties. Moreover, according to this method it is necessary to heat the sand to a temperature of 400.degree.-500.degree.F. The obtained core or moulding sand mixture shows high green strength and only moderate dry strength. Usually, the sand composition contains also an addition of the commonly used bonding clays or other binders and an appropriate amount of water. The fact that the sand must be heated as well as its low dry strength detract from its value.